Prop Calculations

Discussion in 'Props' started by Mik the stick, Oct 21, 2014.

  1. Mik the stick
    Joined: Dec 2012
    Posts: 189
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    Location: Devon

    Mik the stick Senior Member

    I used Dave Gerr's Prop book to predict the required prop for Molakai Straight as a test and practice for the design I am working on. I Picked MS because it had all the parameters I would need. Gerr predicts 375SHP
    Min Prop Dia was about 44" and the 440hp engine can turn 37" Gearbox is 2.43 to 1. Even a 44" wide blade prop was a bit down on blade area. I am sure My calculations were right, perhaps there was a misprint 2.43 instead of 3.43.
    So I started to look at military boats again because I have a lot of info.

    Gerrs formula for predicting power seems way off, (but it's not). A 2940ton Fletcher has 60,000SHP Gerrs formula predicts 38,756SHP for 37kts. but that formula assumes a prop of 55% eff. 38,756SHP is for average conditions for heavy weather 38756/1.36 x 2.5 = 71242SHP but prop eff is 82.1%. If the ship was making 5kts prop eff would drop to 61.1% and 3kts steerage way 57.2%. working out the propulsive coefficient of the destroyer it is 0.63. if Gerrs prop eff was really propulsive coefficient the Fletcher needs 33,850SHP in average conditions and 62,224SHP in heavy weather. In calculating the PC 15% was allowed for appendages, this is probably to high, with a 10% allowance power would be nearer 60,000SHP.

    What the above paragraph proves to me is when you use power as a starting point in prop calculations predicted power is not the reason the prop is too small. Using the formulas to predict props for ships like the Bangor class minesweeper the predicted props are a slightly different pitch and diameter but very close to what is actually fitted. So there is not much wrong with the other formulas in the book.

    In conclusion I don't believe the Molokai Strait has a 2.43 to 1 gearbox. I would be interested to know the actual gearbox/prop combination. Perhaps someone could provide info on a boat which includes engine/gearbox/prop combination so I can go throw the calculations step by step.
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